arXiv subjects

B. A. Kniehl

Publications and source records attributed to B. A. Kniehl.

At least 55 records · Page 3Linked to original sources

arXiv2014

Prompt-photon plus jet associated photoproduction at HERA in the parton Reggeization approach

We study the photoproduction of isolated prompt photons associated with hadron jets in the framework of the parton Reggeization approach. The cross section distributions in the transverse energies and pseudorapidities of the prompt photon and the jet as well as the azimuthal decorrelation variables measured by the H1 and ZEUS collaborations at DESY HERA are nicely described by our predictions. The main improvements with respect to previous studies in the $k_T$-factorization framework include the application of the Reggeized-quark formalism, the generation of exactly gauge-invariant amplitudes with off-shell initial-state quarks, and the exact treatment of the $γR\to γg$ box contribution with off-shell initial-state gluons.

hep-ph↗

arXiv2013

Gluon- and Quark-Jet Multiplicities with NNNLO and NNLL Accuracy

We present a new approach to consider and include both the perturbative and the non-perturbative contributions to the multiplicities of gluon and quark jets. Thanks to this new method, we have included for the first time new contributions to these quantities obtaining next-to-next-to-leading-logarithmic resummed formulas. Our analytic expressions depend on two non-perturbative parameters with a clear and simple physical interpretation. A global fit of these two quantities shows how our results solve a longstanding discrepancy in the theoretical description of the data.

hep-ph↗

arXiv2012

Production of one or two vector mesons in peripheral high-energy collisions of heavy ions

We study the production of spin-one mesons in high-energy heavy-ion collisions with peripheral kinematics in the framework of QED. The cross sections of the production of a single vector meson and of two different ones are presented. The explicit dependence on the virtuality of the intermediate vector meson is obtained within a quark model. The effect of reggeization of the intermediate vector meson state in the case of the production of two vector mesons is taken into account.

hep-ph↗

arXiv2012

Inclusive Charmed-Meson Production at the CERN LHC

We present predictions for the inclusive production of charmed hadrons at the CERN LHC in the general-mass variable-flavor-number scheme at next-to-leading order. Detailed numerical results are compared to data where available, or presented in a way to ease future comparisons with experimental results. We also point out that measurements at large rapidity have the potential to pin down models of intrinsic charm.

hep-ph↗

arXiv2011

Timelike Single-logarithm-resummed Splitting Functions

We calculate the single logarithmic contributions to the quark singlet and gluon matrix of timelike splitting functions at all orders in the modified minimal-subtraction (MSbar) scheme. We fix two of the degrees of freedom of this matrix from the analogous results in the massive-gluon regularization scheme by using the relation between that scheme and the MSbar scheme. We determine this scheme transformation from the double logarithmic contributions to the timelike splitting functions and the coefficient functions of inclusive particle production in e+ e- annihilation now available in both schemes. The remaining two degrees of freedom are fixed by reasonable physical assumptions. The results agree with the fixed-order results at next-to-next-to-leading order in the literature.

hep-ph↗

arXiv2011

Inclusive B-Meson Production at the LHC in the GM-VFN Scheme

We calculate the next-to-leading-order cross section for the inclusive production of B mesons in pp collisions in the general-mass variable-flavor-number scheme, an approach which takes into account the finite mass of the b quarks. We use realistic evolved non-perturbative fragmentation functions obtained from fits to e+e- data and compare results for the transverse-momentum and rapidity distributions at a center-of-mass energy of 7 TeV with recent data from the CMS Collaboration. We find good agreement, in particular at large values of pT.

hep-ph↗

arXiv2011

The Epsilon Expansion of Feynman Diagrams via Hypergeometric Functions and Differential Reduction

Higher-order diagrams required for radiative corrections to mixed electroweak and QCD processes at the LHC and anticipated future colliders will require numerically stable representations of the associated Feynman diagrams. The hypergeometric representation supplies an analytic framework that is useful for deriving such stable representations. We discuss the reduction of Feynman diagrams to master integrals, and compare integration-by-parts methods to differential reduction of hypergeometric functions. We describe the problem of constructing higher-order terms in the epsilon expansion, and characterize the functions generated in such expansions.

math-ph↗

arXiv2011

Single jet and prompt-photon inclusive production with multi-Regge kinematics: From Tevatron to LHC

We study single jet and prompt-photon inclusive hadroproduction with multi-Regge kinematics invoking the hypothesis of parton Reggeization in t-channel exchanges at high energy. In this approach, the leading contributions are due to the fusion of two Reggeized gluons into a Yang-Mills gluon and the annihilation of a Reggeized quark-antiquark pair into a photon, respectively. Adopting the Kimber-Martin-Ryskin and Bluemlein prescription to derive unintegrated gluon and quark distribution functions of the proton from their collinear counterparts, for which we use the Martin-Roberts-Stirling-Thorne set, we evaluate cross section distributions in transverse momentum (p_T) and rapidity. Without adjusting any free parameters, we find good agreement with measurements by the CDF and D0 Collaborations at the Tevatron and by the ATLAS Collaboration at the LHC in the region 2p_T/sqrt{S} < 0.1, where sqrt{S} is the hadronic c.m. energy.

hep-ph↗

arXiv2011

Timelike small x Resummation for Fragmentation Functions

The status of small x resummation in the timelike kinematics is discussed. We present a general procedure to extract the large logarithms of x in the MS factorization scheme and to resum them in a closed form. New results for the doubly-logarithm-resummed coefficient functions will be reviewed. All our resummation formulae are in agrement with the fixed NNLO computations recently done by other groups in the \bar{MS} scheme.

hep-ph↗

arXiv2011

Fully double-logarithm-resummed cross sections

We calculate the complete double logarithmic contribution to cross sections for semi-inclusive hadron production in the modified minimal-subtraction scheme by applying dimensional regularization to the double logarithm approximation. The full double logarithmic contribution to the coefficient functions for inclusive hadron production in electron-positron annihilation is obtained in this scheme for the first time. Our result agrees with all fixed order results in the literature, which extend to next-next-to-leading order.

hep-ph↗

arXiv2011

Differential Reduction Techniques for the Evaluation of Feynman Diagrams

Stable reduction methods will be important in the evaluation of high-order perturbative diagrams appearing in QCD and mixed QCD-electroweak radiative corrections at the LHC. Differential reduction techniques are useful for relating hypergeometric functions with shifted values of the parameters. We present a proposition relating the number of master integrals in the expansion of a Feynman diagram to the number of derivatives in a differential reduction.

math-ph↗

arXiv2011

Supersymmetric QCD corrections to $e^+e^-\to t\bar{b}H^-$ and the Bernstein-Tkachov method of loop integration

The discovery of charged Higgs bosons is of particular importance, since their existence is predicted by supersymmetry and they are absent in the Standard Model (SM). If the charged Higgs bosons are too heavy to be produced in pairs at future linear colliders, single production associated with a top and a bottom quark is enhanced in parts of the parameter space. We present the next-to-leading-order calculation in supersymmetric QCD within the minimal supersymmetric SM (MSSM), completing a previous calculation of the SM-QCD corrections. In addition to the usual approach to perform the loop integration analytically, we apply a numerical approach based on the Bernstein-Tkachov theorem. In this framework, we avoid some of the generic problems connected with the analytical method.

hep-ph↗

arXiv2010

Factorization breaking in high-transverse-momentum charged-hadron production at the Tevatron?

We compare the transverse momentum (p_T) distribution of inclusive light-charged-particle production measured by the CDF Collaboration at the Fermilab Tevatron with the theoretical prediction evaluated at next-to-leading order in quantum chromodynamics (QCD) using fragmentation functions recently determined through a global data fit. While, in the lower p_T range, the data agree with the prediction within the theoretical error or slightly undershoot it, they significantly exceed it in the upper p_T range, by several orders of magnitude at the largest values of p_T, where perturbation theory should be most reliable. This disagreement is too large to be remedied by introducing additional produced particles into the calculation, and potentially challenges the validity of the factorization theorem on which the parton model of QCD relies. Clearly, a breakdown of the factorization theorem, being a fundamental property of QCD, would be extremely difficult to understand.

hep-ph↗

arXiv2010

Inclusive b and b anti-b production with quasi-multi-Regge kinematics at the Tevatron

We consider b-jet hadroproduction in the quasi-multi-Regge-kinematics approach based on the hypothesis of gluon and quark Reggeization in t-channel exchanges at high energies. The preliminary data on inclusive b-jet and b anti-b-dijet production taken by the CDF Collaboration at the Fermilab Tevatron are well described without adjusting parameters. We find the main contribution to inclusive b-jet production to be the scattering of a Reggeized gluon and a Reggeized b-quark to a b quark, which is described by the effective Reggeon-Reggeon-quark vertex. The main contribution to b anti-b-pair production arises from the scattering of two Reggeized gluons to a b anti-b pair, which is described by the effective Reggeon-Reggeon-quark-quark vertex. Our anaysis is based on the Kimber-Martin-Ryskin prescription for unintegrated gluon and quark distribution functions using as input the Martin-Roberts-Stirling-Thorne collinear parton distribution functions of the proton.

hep-ph↗

arXiv2009

Orthopositronium lifetime at O(alpha) and O(alpha^3 ln(alpha)) in closed form

Recently, the O(alpha) and O(alpha^3 ln(alpha)) radiative corrections to the orthopositronium lifetime have been presented in closed analytical form, in terms of basic irrational numbers that can be evaluated numerically to arbitrary precision [Phys. Rev. Lett. 101, 193401 (2008)]. Here, we present the details of this calculation and reveal the nature of these new constants. We also list explicit transformation formulas for generalized polylogarithms of weight four, which may be useful for other applications.

hep-ph↗

arXiv2009

$M(η_b)$ and $α_s$ from Nonrelativistic Renormalization Group

We sum up the next-to-leading logarithmic corrections to the heavy-quarkonium hyperfine splitting using the nonrelativistic renormalization group. On the basis of this result, we predict the mass of the $η_b$ meson to be $M(η_b)=9419 \pm 11 {(\rm th)} {}^{+9}_{-8} (δα_s) MeV$. The experimental measurement of $M(η_b)$ with a few MeV error would be sufficient to determine $α_s(M_Z)$ with an accuracy of $\pm 0.003$. The use of the nonrelativistic renormalization group is mandatory to reproduce the experimental value of the hyperfine splitting in charmonium.

hep-ph↗

arXiv2009

Small-x behavior of the structure function F_2 and its slope partial ln(F_2)/partial ln(1/x) for "frozen" and analytic strong-coupling constants

Using the leading-twist approximation of the Wilson operator product expansion with "frozen" and analytic versions of the strong-coupling constant, we show that the Bessel-inspired behavior of the structure function F_2 and its slope\break partial ln(F_2)/partial ln(1/x) at small values of x, obtained for a flat initial condition in the DGLAP evolution equations, leads to good agreement with experimental data of deep-inelastic scattering at DESY HERA.

hep-ph↗

arXiv2009

Open charm hadroproduction and the intrinsic charm content of the proton

We advocate charmed-hadron inclusive hadroproduction as a laboratory to probe intrinsic charm (IC) inside the colliding hadrons. Working at next-to-leading order in the general-mass variable-flavor-number scheme endowed with non-perturbative fragmentation functions recently extracted from a global fit to e^+e^- annihilation data from KEKB, CESR, and LEP1, we first assess the sensitivity of Tevatron data of D^0, D^+, and D^{*+} inclusive production to the IC parameterizations provided by Pumplin et al. We then argue that similar data from pp collisions at RHIC would have the potential to discriminate between different IC models provided they reach out to sufficiently large values of transverse momentum.

hep-ph↗